What stops a teacher from giving unlimited points to their House?

Question

In the end of Order of Phoenix, Prof. McGonagall gives Luna, Harry, Neville, Ron, and Hermione 50 points each and the "House point meter" immediately refills itself. If the meter would immediately refill itself when a teacher gives a command, what stops a teacher from giving their House points now and then, which isn't technically wrong. What happens if a teacher does it, and are there any rules which stop them from doing so?

Also shouldn't teachers not be allowed to give their own houses points (like stackexchange) as it can be easily exploited

“Ah. I see there are no longer any points left in the Gryffindor hourglass to take away. In that case, Potter, we will simply have to —”
“Add some more?” Professor McGonagall had just stumped up the stone steps into the castle. She was carrying a tartan carpetbag in one hand and leaning heavily on a walking stick with her other, but otherwise looked quite well.

“So that’s fifty each for Potter, the two Weasleys, Longbottom, and Miss Granger,” said Professor McGonagall, and a shower of rubies fell down into the bottom bulb of Gryffindor’s hourglass as she spoke. “Oh — and fifty for Miss Lovegood, I suppose,”

Answer 1

Short answer: Nothing stops the teachers from exploiting the point system as they wish.

The purpose of the point system is to introduce a sense of community between students and evoke motivation to learn and behave well. As Profesor McGonagall explained in the first book:

While you are at Hogwarts, your triumphs will earn your house points, while any rulebreaking will lose house points. At the end of the year, the house with the most points is awarded the house cup, a great honor. I hope each of you will be a credit to whichever house becomes yours.

The point system is basically the tool for the teachers and prefects to create social pressure on the students. The main idea behind is that one would be respected and praised by her/his peers whenever they do good and are awarded points. Similarly, they would be disliked and subject to ostracism if they misbehave and lose points. That was exactly what happened to Harry, Hermione, and Neville after they lost 150 points the night they left Norbert to Charlie's friends:

From being one of the most popular and admired people at the school, Harry was suddenly the most hated. Even Ravenclaws and Hufflepuffs turned on him, because everyone had been longing to see Slytherin lose the house cup. Everywhere Harry went, people pointed and didn’t trouble to lower their voices as they insulted him. Slytherins, on the other hand, clapped as he walked past them, whistling and cheering, “Thanks Potter, we owe you one!”

(...)

Hermione and Neville were suffering, too. They didn’t have as bad a time as Harry, because they weren’t as well-known, but nobody would speak to them, either. Hermione had stopped drawing attention to herself in class, keeping her head down and working in silence.

The system works great for that purpose. In general, the greater effect the teacher wants to invoke, the greater number of points they award or withhold. But no particular limitation for teachers is mentioned in the books. Professor McGonagall said once that Umbridge as a teacher had every right to give detention and I assume that was also true for the house points:

“Every evening this week!” Harry repeated, horrified. “But, Professor, couldn’t you — ?”

“No, I couldn’t,” said Professor McGonagall flatly.

“But —”

“She is your teacher and has every right to give you detention. (...)"

Throughout the books we can find a lot of examples of teachers using arbitrary numbers, mostly Snape:

Ron finally cracked and flung a large, slippery crocodile heart at Malfoy, which hit him in the face and caused Snape to take fifty points from Gryffindor.

But other teachers also have no problem in bending the rules, like Professor Sprout:

The teachers were, of course, forbidden from mentioning the interview by Educational Decree Number Twenty-six, but they found ways to express their feelings about it all the same. Professor Sprout awarded Gryffindor twenty points when Harry passed her a watering can;

The teachers report to Headmaster which, I guess, should act whenever a teacher is manifestly unjust. But we know that Dumbledore also likes to play with the numbers for his own benefit:

The din was deafening. Those who could add up while yelling themselves hoarse knew that Gryffindor now had four hundred and seventy-two points — exactly the same as Slytherin. They had tied for the house cup — if only Dumbledore had given Harry just one more point. Dumbledore raised his hand. The room gradually fell silent. “There are all kinds of courage,” said Dumbledore, smiling. “It takes a great deal of bravery to stand up to our enemies, but just as much to stand up to our friends. I therefore award ten points to Mr. Neville Longbottom.”

Answer 2

The only thing that stops them is the thing that keeps instructors from being unfair with points in our own world -- they see the house point structure as being a useful teaching device. And, since they are teachers and have internal motivation to teach, they use the device as it is intended.

Obviously, you can quarrel with particular individuals (cough, cough, Snape) using the device in unfair ways. Just as a few real-world teachers may grade the work of their class pet a bit leniently. But, overall, the system mostly works.